1. Field of the Invention
The present invention pertains to the field of ovens generally. More particularly, this invention is specifically usable as a construction for those ovens used for reflow soldering of electrical or other components to a printed circuit board positioned therewithin or conveyed therethrough.
Normally a printed circuit board will include a plurality of components positioned thereon for soldering in a predetermined pattern. A paste mixture of flux and solder is located at each position where a soldered electrical connection needs to be created between the component and the board. At least one separate heating chamber is included within a reflow oven and often as many as twelve or more separate heating chambers are utilized. The use of configurations having three heating chambers is quite common. These multiple heating environments allow each chamber to be maintained at different operating temperatures for providing the capability of designing a unique heating profile for the printed circuit boards as they pass through the entire solder reflow oven apparatus. In those systems using only a single heating chamber it is not uncommon to provide for batch heating of the articles to be heated instead of using a conveyor traveling therethrough.
Initially the printed circuit board is pre-heated up to a temperature sufficient for activation of the flux. The board temperature is then maintained at this activation temperature for a preset period of time, often defined as the “dwell” period, in order to allow the flux to become activated for preparing the surfaces of the electrical components and the printed circuit boards in order to create a soldered connection therebetween having very low resistance to current flow.
After the flux is fully activated for the dwell period, the printed circuit board is then heated to at least as high or greater than the melting temperature of the solder in order to initiate soldering of the leads of the electrical component to the printed circuit board at all required locations thereon. The temperature is maintained at a level at least as high as the solder melting temperature for a significant period of time in order to achieve fully conductive electrical connection between the leads of the electrical components and the attachment locations on the printed circuit board. Thereafter the board exits from the oven apparatus and is subsequently cooled.
The state of the art of oven convection flow technology has advanced significantly over the years. Also, the more specific technology associated with reflow ovens has also advanced to a great extent recently. In the original technology, the contacts of the electrical components extended through holes defined in the printed circuit board and this design was designated “through hole” technology. Thereafter, approximately twenty years ago more widespread use of surface mount technology was initiated wherein circuit patterns were defined on the printed circuit board by selective metallic deposition of circuit patterns directly on the printed circuit board and the components were attached at contact locations without any use of “through holes”.
Historically the electrical lead lines from the components were positioned about the outer periphery of the component itself. More recently, however, these leads have been defined extending outwardly from the undersurface of the individual electrical components in various manners. One of the more common designs is the use of a ball grid array defined on the undersurface of the components. This configuration has significantly increased the difficulty of providing effective contact between the components and the printed circuit board circuitry since the contacts themselves are shielded or sandwiched in between the undersurface of the electrical component and the upper surface of the printed circuit board. The present invention provides a design which is particularly effective with ball grid array technology as well as more conventional surface mounted technology and electrical components utilizing J-leads, gull-wing leads or butt mountings.
Previously, various other heating technologies, such as infrared heating, were the preferred manners of raising the temperature within an oven such as a solder reflow oven. However recently the use of convection heating methods have become more widespread. Convection heating has a distinct advantage of providing a heating process which is not dependent on the color of the surface being heated.
However, infrared technology, since it is responsive to the various emissive characteristics of the components such as the color thereof, has some difficulty in achieving equalized heating of all areas of the circuit board and the electrical components positioned thereon. Convection heating does not have this problem. Also infrared heating emits the energy that causes heating by radiation. This energy transfer tends to create shadowing wherein cool spots can occur especially in those areas immediately adjacent to taller or large electrical components wherein the height of the electrical component might tend to shield certain areas needing to be soldered on the printed circuit board from the infrared heating source. This shadowing has proven to be a great problem in regard to equalizing of heating across the entire printed circuit board especially when conveyed through the oven. In particular smaller and shallower components which are located in close proximity to larger or taller ones are often exposed to less radiation. As such, convection is rapidly replacing infrared processes and is becoming the preferred method for solder reflow ovens currently.
The use of convection heating has been established as being a primary step toward achieving more equalized heating and articles in ovens generally. In particular, convection heating is useful for equalizing heating across the printed circuit board during the various several stages of the solder reflow process. If a significant temperature difference exists between any two areas of the printed circuit board or the electrical components thereof, then it is necessary to see that the entire temperature within the oven is raised such that the lowest temperature experienced therewithin is sufficiently high in order to achieve full reflow soldering of an electrical contact at this point. This process results in the overheating of certain areas which can lead to warping of the printed circuit board and also wastes much energy. Also wider heating parameter tolerances are required because of the wider range of temperature differences created across various points on the printed circuit board during reflow soldering. This widening of the parameter window is even more of a difficult problem in recent years since with the decreased usage of lead-based soldering materials and increased usage of silver-based soldering materials. Thus, it is particularly advantageous at this point to provide a reflow solder oven apparatus and a method of use thereof wherein the temperature differences at various locations across the printed circuit board and the components thereof is maintained at a very low level, preferably lower than two degrees Celsius.
In standard convection ovens the air is heated and then circulation occurs therewithin toward the article to be heated which may be a printed circuit board. These articles or printed circuit boards can be batch heated or can be conveyed through the internal environment of the oven. Normally movement of heated air is directed perpendicularly and downwardly toward the top of the printed circuit board or perpendicularly and upwardly toward the bottom of the circuit board. In other configurations, the air flow for achieving convective heat transfer is achieved by random turbulent flow of air throughout the heating environment. Both of these processes lead to an increase in temperature in the center area of the printed circuit board.
Generally, ovens used for all purposes tend to be warmer at the most central location and cooler at positions radially outward therefrom. Normally ovens are coolest at the outer wall thereof. This temperature difference creates a problem which has been shown to be very difficult to overcome effectively, and is a natural result of the physics of heat dissipation outwardly from the oven in all radial directions. This can be somewhat controlled by the use of effective insulation. However, the temperature differential between the center portion of the oven to the outer periphery thereof will always be significant due to the continual loss of heat outwardly radially from any oven.
Also, as air is directed toward the articles to be heated, such as printed circuit boards, it initially impacts the central area thereof. This air is heated and effectively heats the central areas of the printed circuit boards. The printed circuit boards will be significantly cooler than the surrounding environment prior to entry into the heated oven environment and, as such, will significantly lower the temperature of the heated air after impacting the central area thereof. This air will then move downwardly and outwardly horizontally along the upper surface of the printed circuit board. As the air moves along the surface of the printed circuit board it will dissipate heat slowly since it is raising the temperature of the board as it flows therealong approximately parallel across the surface. When the air reaches the outer periphery of the board, the temperature of the air is significantly reduced which lowers the temperature to which it can raised the outer regions of the boards. This effect will establish a temperature gradient across the printed circuit board from the central regions to the peripheral regions thereof. The printed circuit board will have high temperatures in the central portion and lower temperatures as you move outwardly in all directions radially along the surface of the printed circuit board. This same overheating problem will occur when heat is directed approximately perpendicularly to the conveyed circuit boards from below.
Velocity of air movement is also an important consideration in solder reflow oven design. The increased velocity or different velocities of heated air accelerates the efficiency of conduction of heat to the electrical component and the printed circuit board. However, excessive velocities can create other problems such as threatening to unseat components from their precise positions. Air movement devices which blow heat air directly onto the surface of a printed circuit board, using air flow jets or nozzles, must be very accurately controlled as to velocity in order to move fast enough to transfer a significant amount of heat to the board while at the same time being slow enough in order to prevent movement or damaging of components positioned thereon. This design requirement has the effect of introducing another parameter that must be closely monitored and controlled which provides another area requiring an operating tolerance which can be a problem.
2. Description of the Prior Art
Various configurations for ovens and methods of use thereof have been patented or otherwise disclosed which can achieve equalized heating across the expanse of articles located therewithin or passing therethrough such as shown in U.S. Pat. No. 4,434,845 patented Mar. 6, 1984 to D. C. Steeb on a “Stacked-Plate Heat Exchanger”; and U.S. Pat. No. 4,978,836 patented Dec. 18, 1990 to W. Dieudonne et al and assigned to Heraeus Quarzschmeize GmbH on a “Continuous Oven”; and U.S. Pat. No. 4,990,402 patented Feb. 5, 1991 to G. Kneringer et al and assigned to Schwarzkopf Development Corporation on a “Process For The Manufacture Of A Soldered Joint”; and U.S. Pat. No. 5,193,735 patented Mar. 16, 1993 to I. M. Knight and assigned to Knight Electronics, Inc. on a “Solder Reflow Oven”; and U.S. Pat. No. 5,515,605 patented to H. Hartmann et al on May 14, 1996 and assigned to Robert Bosch GmbH on an “Apparatus And Process For Soldering Component Onto Boards”; and U.S. Pat. No. 5,647,529 patented Jul. 15, 1997 to H. F. Liebman et al and assigned to Motorola, Inc. on a “Method Of Controlling The Temperature Of A Portion Of An Electronic Part During Solder Reflow”; and U.S. Pat. No. 5,739,053 patented Apr. 14, 1998 to T. Kawakita et al and assigned to Matsushita Electric Industrial Co., Ltd. on a “Process For Bonding A Semiconductor To A Circuit Substrate Including A Solder Bump Transferring Step”; and U.S. Pat. No. 5,795,147 patented Aug. 18, 1998 to N. Saxena et al and assigned to The BOC Group, Inc. on a “Furnace Having Regulated Flow Rate Of Inerting Gas”; and U.S. Pat. No. 5,814,789 patented Sep. 29, 1998 to B. O'Leary et al and assigned to BTU International, Inc. on a “Forced Convection Furnace Gas Plenum”; and U.S. Pat. No. 5,860,583 patented Jan. 19, 1999 to H. F. Liebman et al and assigned to Motorola, Inc. on an “Evaporative Cooling Vessel For controlling The Temperature Of A Portion Of An Electronic Part During Solder Reflow”; and U.S. Pat. No. 5,888,102 patented Mar. 30, 1999 to J. Strickland on a “Surface Mount Carrier For Electronic Components”; and U.S. Pat. No. 5,911,486 patented Jun. 15, 1999 to S. J. Dow et al and assigned to Conceptronic, Inc. on a “Combination Product Cooling And Flux Management Apparatus”; and U.S. Pat. No. 5,913,589 patented Jun. 22, 1999 to S. J. Dow and assigned to Conceptronics, Inc. on a “Combination Product Cooling And Flux Management Apparatus”; and U.S. Pat. No. 5,919,317 patented Jul. 6, 1999 to A. Tanahashi et al and assigned to Nippondenso Co., Ltd. on “Soldering Flux, Soldering Paste And Soldering Method Using The Same”; and U.S. Pat. No. 6,015,966 patented Jan. 18, 2000 to J. Rehm and assigned to Rehm Anlagenbau GmbH & Co. on a “Circuit Board Heating Apparatus; and U.S. Pat. No. 6,084,214 patented Jul. 4, 2000 to St. Tallman et al and assigned to Conceptronic, Inc. on a “Reflow Solder convection Oven Multi-Port Blower Subassembly”; and U.S. Pat. No. 6,106,281 patented Aug. 22, 2000 to P. Materna on a “Method Of Reducing The Flow Of Gas Needed For A Chamber With Controlled Temperature And Controlled Composition Of Gas”; and U.S. Pat. No. 6,123,250 patented to F. DeKlein et al on Sep. 26, 2000 and assigned to Soltec B. V. on a “Reflow Oven”; and U.S. Pat. No. 6,129,256 patented Oct. 10, 2000 to J. R. Watson et al and assigned to Intel Corporation on a “Reflow Furnace For An Electronic Assembly”; and U.S. Pat. No. 6,138,893 patented Oct. 31, 2000 to D. V. Caletka et al and assigned to International Business Machines Corporation on a “Method For Producing A Reliable BGA Solder Joint Interconnection”; and U.S. Pat. No. 6,142,363 patented Nov. 7, 2000 to A. Tanahashi et al and assigned to Nippondenso Co., Ltd. on a “Soldering Method Using Soldering Flux And Soldering Paste”; and U.S. Pat. No. 6,146,448 patented Nov. 14, 2000 to R. G. Shaw et al and assigned to Soltec B. V. on a “Flux Management System For A Solder Reflow Oven”; and U.S. Pat. No. 6,157,002 patented Dec. 5, 2000 to W. S. Schjerven et al and assigned to Middleby Cooking Systems Group on a “Small conveyor Toaster/Oven”; and U.S. Pat. No. 6,226,864 patented May 8, 2001 to M. Muziol et al and assigned to Heraeus Electro-Nite International N. V. on a “Process For Producing Printed Circuit Boards With At Least One Metal Layer, Printed Circuit Board And Use Thereof”; and U.S. Pat. No. 6,230,956 patented May 15, 2001 to J. Farroni et al and assigned to STMicroelectronics S.A. on a “Soldering Conveyor Support”; and U.S. Pat. No. 6,236,029 patented May 22, 2001 to M. Leurquin and assigned to Alcatel on an “Apparatus For Soldering Flat Rectangular Connectors And Method Using Same”; and U.S. Pat. No. 6,276,593 patented Aug. 21, 2001 to I. A. Artaki et al and assigned to Agere Systems Guardian Corp. on an “Apparatus And Method For Solder Attachment Of High Powered Transistors To Base Heatsink”; and U.S. Pat. No. 6,289,715 patented Sep. 18, 2001 to C. Gilbert et al and assigned to L'Air Liquide, Societe Anonyme pour l'Etude et l “Exploitation des Procedes Georges Claude on a “System For Analyzing The Atmosphere In An Enclosure For Soldering Or Tinning By Means Of A Metal Alloy”; and U.S. Pat. No. 6,293,455 patented Sep. 25, 2001 to D. V. Culetka et al and assigned to International Business Machines Corporation on a “Method For Producing A Reliable BGA Solder Joint Interconnection”; and U.S. Pat. No. 6,320,165 patented Nov. 20, 2001 to D Z. Ovadia and assigned to Pizza Hut, Inc. on “Impingement Oven Airflow Devices And Methods”; and U.S. Pat. No. 6,323,462 patented Nov. 27, 2001 to D. Strand and assigned to Wisconsin Oven Corporation on a “Conveyor Oven Usable As Pre-Bake Oven In A Print Plate Imaging And Processing System And Method Of Using Same”.